The Effective Diffusion Coefficient of Dissolved Oxygen and Oxidation Rate of Pyrite by Dissolved Oxygen in Compacted Purified and Crude Sodium Bentonites in Carbonate Buffered Solution

Immediately after the geological disposal of high-level radioactive waste, the oxygen initially existing in the repository is expected to strongly affect the redox condition of the near field. The oxygen dissolves in the groundwater, is transported by diffusion through it, and is consumed by the oxidation of pyrite as an impurity in bentonite. To assess the influence of the oxygen, this study was conducted to estimate the diffusion of dissolved oxygen (DO) and the rate of pyrite oxidation by DO in compacted purified and crude sodium bentonites (SBs) in more detail than the Manaka et al. study. The effective diffusion coefficient (De) of DO in the compacted purified SB was measured in low ionic strength solution (carbonate buffered solution with pH ~ 9) using the electrochemical method. The empirical equation between De value of DO and dry density (0.5 × 10³-1.8 × 10³ kg m^-3) of purified SB was obtained as follows:

De_DO^Kunipia-F = 8.2 ± 1.5 × 10^-10

× exp(-2.6 ± 0.2 ×10^-3,

where De_DO^Kunipia-F is the De of DO in compacted purified SB (Kunipia F) (m² s^-1) and is the dry density of the SB (kg m^-3).

On the other hand, the De value of DO in the compacted crude SB was estimated using the relationship between De values of tritiated water in compacted purified and crude SBs. The empirical equation between the De value of DO and dry density (0.5 × 10³-1.8 × 10³ kg m^-3) of crude SB was derived as follows:

De_DO^Kunigel-V1 = 2.04 × 10^-9 exp(-2.6 × 10^-3),

where De_DO^Kunigel-V1 is the De of DO in compacted crude SB (Kunigel V1) (m² s^-1) and is the dry density of the SB (kg m^-3).

The rates of pyrite oxidation by DO were estimated from the experimental data in pyrite-purified SB systems using the obtained De values of DO. The relation between rate constant (k') of pyrite oxidation by DO and dry density () of the SB was derived as follows:

k' = 3.9 ± 1.1 × 10^-8 exp(-1.3 ± 0.3 × 10^-3),

where k' is the rate constant at pH ~ 9 in compacted purified SB of dry density ranging from 0.8 × 10³ to 1.2 × 10³ kg m^-3.

The rate constants of pyrite oxidation by DO in the compacted crude SB (0.8 × 10³ to 1.2 × 10³ kg m^-3) were also calculated using the estimated De values of DO. In general, the values of rate constants in the crude SB are 1.5 times as large as that in the purified SB.